A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The known apparatus can comprise a collector for collecting radiation from a radiation source. The collector can be arranged to transmit the collected radiation to an illumination system. Such an illumination system can be arranged to provide a projection beam of radiation, which is used to apply a desired pattern onto a target portion of a substrate. The collector can comprise, for example, suitable radiation reflecting mirror shells. Particularly, the collector can be arranged to focus incoming radiation, received from the radiation source, onto a small focusing area or focusing point.
During use, the collector can become contaminated with various substances. For example, in case the radiation source is a Sn radiation source, collector surfaces can be contaminated by tin (Sn) and/or tin-oxide (SnOx) contamination. Such contamination can have a negative effect on the performance of the collector. In a mentioned Sn radiation source, usually, a plasma is formed by heating up Sn atoms. Various methods are known to remove contamination from the collector. However, the known methods are relatively complex, expensive to implement, and relatively inefficient.